In the disclosure that follows, the following acronyms may be used:
AWGNAdditive White Gaussian NoiseBER Bit Error RateBPSKBinary Phase Shift KeyingCPCircular PolarizationDCPDual Circular PolarizedDSAMDirect Spatial Antenna ModulationDSP Digital Signal ProcessorDSSSDirect Sequence Spread SpectrumEVMError Vector MagnitudesGHz GigahertzLAN Local Area NetworkLHCPLeft Hand Circular PolarizedLNA Low Noise AmplifierMHz MegahertzMSP Microstrip PatchPAPower AmplifierPCBPrinted Circuit BoardPEC Perfect Electric ConductorPIN Positive Intrinsic NegativePSDPower Spectral DensityQPSKQuadrature Phase Shift KeyingRFRadio FrequencyRHCPRight Hand Circular PolarizedSNR Signal to Noise RatioSTTRSmall Business Technology Transfer Program
Modern society is increasingly dependent upon digital electronic communications. The electromagnetic spectrum is limited in nature, and hence the use of wireless radio-frequency electronic techniques to achieve the efficient transmission of digital communications is subject to ever increasing demand. There is a fundamental limitation to the number and rate of wireless transmissions that can be supported simultaneously in the finite electromagnetic spectrum. Any means to increase the data rate of a wireless digital transmission over a fixed-width frequency channel (fixed channel bandwidth) without affecting the quality of the transmission is thus highly desirable.
Existing bandwidth-efficient modulation formats make use of amplitude and phase-based techniques exclusively. That is, each data symbol to be transmitted exists as a different manipulation of the amplitude and phase state of the radio-frequency (RF) carrier signal, and the states are changed over time to communicate the data stream. The existing body of technical knowledge, documented in numerous works, is filled with a wide variety of examples that are familiar to one of ordinary skill in the art.
Very common examples of existing techniques are on-off keying (OOK), binary phase-shift keying (BPSK), and quadrature-phase shift keying (QPSK). The core dimensionality of these common temporal-only formats is two: either the amplitude or the relative phase, or both, of the carrier signal can be manipulated to form different symbols as a function of time. In OOK, the carrier is shifted between one of two amplitude levels to form a binary symbol set. In BPSK, the carrier phase is shifted between one of two values to form a binary symbol set. In QPSK, both the amplitude and phase of the carrier are manipulated to form a two-bit symbol set, such that each symbol sent over a given time period represents two bits of information.
Typically, higher order modulation formats are implemented by using more than just two amplitude and/or phase points in a digital transmission scheme. Examples are M-ary Phase shift keying (M-PSK) which is a phase modulation format, M-ary pulse amplitude modulation (M-PAM) which is an amplitude modulation format, and the M-ary quadrature amplitude modulation (M-QAM) format which uses both amplitude and phase together. In all cases, the “M” prefix relates the number of possible symbol states used. For example, 8-PSK would use 45 degrees of carrier phase difference between symbols (equal spacing) to generate a format with three bits per symbol. A 16-QAM format would encode four bits per symbol through the use of a combination of four amplitude states and four phase states.
QPSK represents the previously optimal four-level modulation format with respect to bandwidth efficiency. It is capable of the best bandwidth efficiency, often expressed as bits-per-second-per-Hertz, for a given transmit power and bit error probability. Although capable of packing more bits of information into a fixed bandwidth, all other high-order amplitude and phase based modulation formats suffer from increased error rates in their transmissions as “M” is increased with transmit power held constant.
When carrier amplitude and phase both have the potential to change in each symbol period during transmission, the linearity of the power amplifier used to transmit the signal becomes important in order to produce a non-distorted transmit signal. There is an upper limit to the maximum linear output power and supply efficiency at a given frequency of operation for the type of solid state power amplifier technology currently in use. It is a common requirement that amplifiers used to transmit high-order amplitude and phase modulated signals be “backed off” from their maximum operating output level in order to meet transmit signal distortion requirements, further reducing the maximum output power available to existing systems. These considerations of power amplifier linearity are obvious to anyone practiced in the art of wireless digital communication.
In the context of an electromagnetic radiator, polarization is defined as the instantaneous vector direction of the electric field of the propagating wave from the perspective of the transmit antenna. There are basically two types of polarization, linear and elliptical. In linear polarization, the electromagnetic wave propagating outward from the transmitting antenna exists (and varies in amplitude as a cosinusoid) along a single vector direction. For an elliptically polarized wave, the electric field vector rotates around the axis of propagation as a function of time, tracing out an ellipse as seen from behind. When both orthogonal components of an elliptical wave have the same peak amplitude, then the polarization is said to be circular.
The current paradigm in radio-frequency (RF) electronic communications is heavily weighted toward the utilization of time-based modulation. Each symbol to be transmitted is encoded by expressing a particular amplitude and phase state of the transmitted signal for a particular segment of time, after which a new symbol is expressed, and so on. The current paradigm is of course very effective and highly developed, but nonetheless totally ignores an entire dimension of modulation: space. The spatial modulation dimension ignored in current techniques is exactly that which enables the significant advantages of the present innovation.
Additionally, the prevalent paradigm in RF electronic communications also treats 1) the RF modulating element and 2) the RF antenna as totally separate and distinct system elements. As such, each is designed and defined independently according to “black box” level specifications and connected together in a functionally modular fashion, wherein the baseband data message signal interacts with the RF carrier in the RF modulating element to form a composite signal wholly independent of the characteristics of the RF antenna. The composite modulated RF carrier signal is then provided as a generic input to the RF antenna. This situation is illustrated in FIG. 1A, and represents essentially all existing applications of art.
There are several important aspects of existing approaches to modulation that rely on the architecture illustrated in FIG. 1A. First, the modulator stage is a lossy system component, wherein some of the RF carrier signal power is used up in the modulation process. This loss must be overcome through additional re-amplification of the output of the core modulating element, a function that is often, but not always, included internal to the integrated circuit or sub-system comprising the modulator. The amplification needed to overcome the losses associated with existing modulation techniques requires additional system power supply consumption.
Secondly, the existing architecture class of FIG. 1A requires that the final amplification stage process the composite modulated signal directly as it amplifies the composite signal up to the desired transmit power level prior to being fed to the RF antenna. Linearity performance requirements are thereby imposed on the final power amplification (PA) stage such that a failure to meet the linearity requirements will result in an inability to achieve some desired level of transmit modulation accuracy and thus wireless communications link performance.
A class of technologies utilizes the antenna to modulate a carrier and is sometimes described as using “direct antenna modulation” techniques. These methods tend to focus on amplitude modulation only, and they do not leverage the spatial aspects of the antennas. Other current research efforts that use the term “antenna modulation” do not encode information symbols on a transmitted signal, but are rather attempts to achieve an increase in the equivalent instantaneous impedance bandwidth of an antenna, which is otherwise used in a traditional fashion. In both cases, the antenna is “conditioned” to send data, but the data do not control how the antenna operates.
What would be useful is a modulation and demodulation scheme that achieves an improved data rate at a lower cost and that leverages the spatial aspects of an antenna.